The present invention relates generally to optical systems and, more particularly, to a method and apparatus for pumping fiber Raman amplifiers.
Fiber optic based telecommunication networks are capable of transmitting data over several tens of kilometers without signal amplification. Transmitting data over distances greater than 100 kilometers requires that the signal be amplified. Currently, the two most popular optical amplifiers are erbium doped fiber amplifiers (EDFA""s) and optical fiber amplifiers utilizing the Raman effect.
Regardless of the type of optical amplifier used in an optical network, the network""s signal capacity is limited by the amplifier""s spectral gain width as well as any associated gain non-uniformities. Gain non-uniformities within the utilized gain spectrum result in a non-uniform bit error rate across the network channels. Accordingly, a conventional network utilizing wavelength division multiplexing (WDM) technology requires gain flatness of approximately xc2x10.5 dB.
Today, the typical optical amplifier is an EDFA providing signal amplification over the bandwidth of the gain spectrum of erbium, specifically 1520 to 1560 nanometers. A telecommunication network utilizing an EDFA will transmit data on multiple wavelength channels that lie within the EDFA gain bandwidth. As the demand for data increases, however, the required data rate increases, as does the number of required wavelength channels. Since the gain bandwidth of Raman amplifiers is not intrinsically limited, optical amplifiers based on the Raman effect have recently become the focus for commercial development.
The wavelength at which a Raman amplifier provides gain is determined by the wavelength of its pump laser. Therefore, through appropriate choice of pump wavelength, Raman amplifiers can provide signal amplification for any wavelength channel within the transparency range of an optical fiber.
The principal challenge facing successful commercial deployment of Raman amplifiers is the development of an economical pump laser that provides high power, typically in the range of 1 to 3 Watts, at the desired wavelength in a diffraction limited beam. In the near term, Raman amplifiers will most likely be based on pump lasers operating in the wavelength range of 1400 to 1500 nanometers. Furthermore, due to cost, package size, and efficiency considerations, semiconductor pump lasers are the preferred technology. However, solitary diode lasers operating in the desired wavelength range are currently only capable of producing roughly 200 mW, far less than the 1 to 3 Watt requirement.
In an attempt to overcome this deficiency, WDM has been used to combine the output of several individually packaged diode pump lasers to achieve a higher power pump laser. In the WDM approach, the wavelength of each diode pump laser is controlled individually and their outputs are combined using either dispersive or dichroic optical elements. For example, H. Kidorf et al. disclose a broad bandwidth Raman amplifier utilizing 8 diode laser pumps ranging in wavelength from 1416 to 1502 in an article entitled Pump Interactions in a 100-nm Bandwidth Raman Amplifier, published in IEEE Photonics Technology Letters, vol. 11, no. 5, May 1999.
There are several drawbacks associated with a pump laser system utilizing a conventional WDM approach. First, the cost of these systems is typically quite high as each of the constituent diode lasers is individually packaged, fiber coupled, and temperature controlled. Second, as these systems require a WDM wavelength combiner each time the output from two diode lasers are combined, i.e. a four diode WDM pump requires three beam combination steps and an eight diode WDM pump requires seven beam combination steps, a conventional WDM based system rapidly becomes overly complicated and inefficient. Due to the power loss and increase in complexity that is associated with each beam combination step, increasing pump power using this brute force approach becomes untenable with more than just a handful of diode lasers.
Accordingly, what is needed in the art is a Raman optical amplifier system with a broad and relatively flat gain bandwidth. The present invention provides such a system.
The present invention provides a method and apparatus for achieving broad gain bandwidth and gain uniformity in a Raman amplifier through the use of a wavelength multiplexed pump source that outputs a high power (e.g.,  greater than 1 Watt), single beam of relatively large bandwidth. The pump source of the invention, in addition to offering high power and broad bandwidth, can be tailored to provide a specific pump profile, thus providing a means to achieve gain flattening within a specific Raman gain bandwidth of the optical amplifier. As a result, a network utilizing a Raman optical amplifier, in accordance with the invention, can accommodate more channels than that achievable in a conventionally pumped system.
In at least one embodiment of the invention, the optical amplifier pump source is comprised of a multi-gain element array within an external resonator. Interposed between the array and the resonator output coupler are a collimating element and a diffraction grating. A refractive optic, a xc2xc pitch GRIN lens, or a reflective optic can be used as the collimating element. The diffraction grating can either be transmissive or reflective. The combination of the diffraction grating and the collimating element forces each emitter within the array to lase at a distinct wavelength. If the gain bandwidth of a single emitter array is less than the desired bandwidth, either multiple arrays of differing center wavelength are packaged together or a large array is used with a laterally varying quantum well thickness or epitaxy. An intracavity spatial filter can be used to improve the beam quality and reduce emitter cross-talk. An external optical element can be used to condition the pump source output beam as necessary.
In at least another embodiment of the invention, the optical amplifier pump source is comprised of the outputs of a pair of multiple gain elements arrays multiplexed within a single resonator cavity. The resonator cavity is comprised of a high reflector, preferably applied to the back facets of the arrays, and an output coupler. Multiplexing can be achieved, for example, with a polarization sensitive beam combiner. Interposed between each array and the output coupler are a collimating optic and a single diffraction grating, both of which can either be transmissive or reflective. The combination of the diffraction grating and the collimating element forces each emitter within each array to lase at a distinct wavelength. Each of the arrays are positioned relative to one another and to the diffraction grating in such a manner as to cause an interlacing of the lasing wavelengths of the individual gain elements of the two arrays. As a consequence, the wavelength separation between spectrally adjacent lasers can be further reduced as necessary to achieve the desired pump profile. Each array can be comprised of multiple arrays of differing center wavelength packaged together or of a single, large array with a laterally varying quantum well thickness or epitaxy. An intracavity spatial filter can be used to improve the beam quality and reduce emitter cross-talk. An external optical element can be used to condition the pump source output beam as necessary.